Question: Which unit of measurement measures density as a physical quality?
Paraphrase: What is the SI unity of density?
SPARQL (Wikidata):  select distinct ?sbj where { ?sbj wdt:P111 wd:Q29539 . ?sbj wdt:P31 wd:Q10387685 } 
Answer: Planck density, gram per litre, kilogram per cubic metre, gram per cubic centimetre, milligram per cubic metre, gram per cubic metre, milligram per litre